In a double boiler or a bowl set over a pot of water, combine the milk and marshmallows. Heat, stirring occasionally to mash marshmallows, until marshmallows are partially melted. Add Hershey Bars and stir often until everything is melted. Remove from heat and let cool completely (about an hour)

In a bowl (preferably metal), whip the cream until fluffy. Stir in the vanilla. Fold the whipped cream into the cooled chocolate mixture, scoop it put all in the pie shell.

Chill for 4 hours or even better, overnight. If you are using a smallish store-bought graham cracker crust, it may not all fit.